The present invention relates to a perpendicular magnetic recording medium, which utilizes remanent magnetism, in a direction perpendicular to its plane, for improving its recording density.
As a typical example of the perpendicular magnetic recording medium of this type, there is a floppy disk in which a thin film of a cobalt-chromium alloy, having an axis of easy magnetization in a direction perpendicular to its plane, is formed as a magnetic recording layer on a base, by batched or continuous sputtering. The base is formed of a heat-resistant polymer film, such as polyimide or polyethylene terephthalate. The recording layer is formed of a cobalt-chromium alloy film, or an oxide film having a magnetoplumbite-type crystal structure, such as barium ferrite or strontium ferrite. In a recording system using floppy disks, the medium rotates generally at a speed of 300 to 600 rpm. Reading or writing operations are performed by means of a magnetic head in contact with the medium, utilizing the effect of electromagnetic exchange. When using the floppy disks in a perpendicular magnetization system for higher recording density, especially linear recording density, it is necessary to reduce the distance between the magnetic head and noncontacting surface portions, attributable to the irregularities of the surface of the medium which is traced by the head. Although generally regarded as even or nearly-even, the surface of the medium actually has very fine indentations. The shape of these indentations greatly influences the characteristics of the medium, including the durability, dropout, linear recording density, and stick.
However, the surface roughness of the medium for high recording density is grasped only very vaguely. For example, recording media have been proposed whose arithmetical mean deviation of the surface roughness profile Ra and maximum projection height Rmax are highly regulated. The media may be apparently equivalent in values Ra and Rmax, within a predetermined range of measurement. However, a duration test indicates some variations in characteristics of the media. Material dust of the media or protective layer, attributable to abrasion, accumulates on the magnetic head, thus widening the space between the media and head, and possibly damaging the medium surface. As a result, the media are lowered in life performance, as well as in reading and writing characteristics. In particular, the damage to the media is subject to substantial variations, resulting in lower reliability.
Moreover, the perpendicular magnetic recording system for high-density recording is confronted by some technical problems to be solved before it can start to be used practically. Primarily, the surface of the magnetic layer of the media must be brought more closely into contact with the magnetic head. In the recording system of this type, the irregularity of the media surface is liable to lower the frequency response, and cause reduction or change of output during writing.
If the medium surface is too smooth, however, the magnetic head will stick to the medium, thus losing its mobility. Thus, the perpendicular magnetic layer cannot be used practically, due to such antinomy, as well as the microscopic configuration of the medium surface, which can not easily be detected by conventional perpendicular measuring means.